To increase durability of power transmission belts, a cord retaining layer in which a cord is embedded, and a rubber layer provided on the inner side or outer side of the cord retaining layer have been made of rubber compositions having different elastic moduli.
Patent Document 1 discloses a V-belt in which a rubber layer having a higher elastic modulus than a cord retaining layer is provided on the outer side of the cord retaining layer; on the other hand, a rubber layer having a lower elastic modulus than the cord retaining layer is provided on the inner side of the cord retaining layer.
Patent Document 2 discloses a toothed belt which has a cord retaining layer made of a rubber composition in which short fibers are oriented in a belt thickness direction, and a toothed rubber layer provided on the inner side of the cord retaining layer and made of a rubber composition not including short fibers.
Patent Document 3 discloses a flat belt in which a rubber layer mixed with short fibers is provided between a cord retaining layer and an inner rubber layer provided on the belt inner side of the cord retaining layer and serving as a pulley contacting portion.
Patent Document 4 discloses a flat belt having a reinforcement fabric between a cord retaining layer made of a thermoplastic resin and each of surface layers provided on both surfaces of the cord retaining layer.    Patent Document 1: Japanese Patent Publication No. S 61-286637    Patent Document 2: Japanese Patent Publication No. 2005-180486    Patent Document 3: Japanese Examined Utility Model Application Publication No. H06-047155    Patent Document 4: Japanese Patent Publication No. 2001-153186